Gas discharge lamps, as are used, for example, for video projection applications, contain a pair of electrodes made of tungsten as an essential feature. In the case of a suitable operating mode, small tips grow on these electrodes, which are used as a starting point for the discharge arc. Such tips have an advantageous effect on the performance of the lamp, in particular with regard to higher luminance, lower tendency to flicker, and lower tendency to burn back.
A stabilization of the tips is accordingly of great significance to implement the above-mentioned advantages. The requirement is to keep stable both the geometry and also the position of the tips on the electrode head, and to do so over the entire service life of the lamp. An electrode reaches temperatures in the vicinity of the melting point of tungsten during the lamp operation on its frontmost end facing toward the discharge arc. For this reason, material is continuously vaporized from the tip. This vaporized material must be supplied to the electrode tip again by a suitable operating mode from the front region of the electrode head.
In addition to these requirements for the electrode stabilization, the lamp operating mode must be closely adapted to the predefined customer applications. In particular in DLP projectors, precise synchronization with the color wheel conventionally used therein must occur. FIG. 1 shows in this context a typical color wheel 10 used in DLP projectors, which in the present case has six different color segments 12a to 12f. As can be inferred from the illustration in FIG. 1, the individual color segments, which are assigned different colors, can have different lengths. In addition, a modulation of the lamp current level synchronized with the color wheel (Unishape principle) is used as a standard feature in present DLP projectors, which results in an improvement of the useful light with regard to maximum achievable brightness, quality of the color reproduction, or white balance.
The current-over-time curves I(t), which have heretofore been used for operating the discharge lamps, and which are also referred to as “current waveform” hereafter, are generally concentrated on the requirements with respect to the modulation of the current level, i.e., with respect to the above-mentioned improvement of the useful light. A chronological modulation, corresponding to a frequency modulation, was heretofore avoided as much as possible. Different current waveform patterns result depending on the color wheel type, in particular depending on the number and the length of the color segments, and its rotation speed, which is typically 120 Hz or 180 Hz. The number and position of the current commutations is conventionally selected so that the resulting current waveform is as symmetrical as possible and a mean lamp frequency of 50 Hz to 90 Hz results. A commutation device is conventionally used for the commutation, which frequently consists of electronic switches, which commutate the polarity of the direct current source in the cycle of the rectangular lamp current, as described in greater detail in DE 10 2007 057 772 A1, for example. During the commutation, overshoots cannot be completely avoided in practice. Therefore, the moment at which a commutation is to occur is generally overlaid with the moment at which the color of the light changes to blank out the overshoots. (One exception from this procedure is, for example, a white segment, in which commutation can also be performed within the segment). For this purpose, as described above, a sync signal is provided, which has a sync pulse in synchronization with the color wheel. With the aid of the sync signal, the color change and the commutation of the lamp current are synchronized.
FIG. 2 shows in this context a typical current waveform known from the related art for the color wheel 10 shown in FIG. 1 having six segments and a rotation frequency of 180 Hz. This corresponds to three rotations of the color wheel 10 per image, wherein the image is projected at a repetition rate (frame rate) of 60 Hz. Three commutations were set per image repetition, from which a mean lamp frequency of 90 Hz results. In the case of the current waveform shown in FIG. 2, the position of the commutations was selected so that the resulting current waveform is symmetrical and is therefore free of direct current.
The respective current increase occurring before a commutation in the current waveform, for example, is referred to as a so-called maintenance pulse (see, for example, U.S. Pat. No. 6,586,892 B2) MP. This ensures stronger melting of the respective electrode operating as an anode in the front region, which is then drawn together by the surface tension of the tungsten and cools again after the subsequent commutation. If this method is repeated at corresponding time intervals, a tip slowly forms therefrom. The maintenance pulse is preferably to be before the commutation for effective application in this case.
In a current waveform, furthermore sections which are assigned to specific segments of the color wheel 10 can be increased according to the Unishape principle. Different segments come into consideration in this case depending on the target to be implemented.
For example, if very good brightness is to be achieved, the bright section or sections of the current waveform are increased. If a good color reproduction is to be implemented, the segments are increased which are present less in the spectrum of the discharge lamp, for example, blue or red. In the current waveform shown in FIG. 2, a corresponding current pulse SP is shown as an example.
Since the mean power is predefined independently of the existing current pulses SP or maintenance pulses MP, the amplitude of other regions of the current waveform is to be reduced when current increases SP or MP are provided. An increase SP extends over an entire segment 12, because of which a current waveform is to be designed for a specific color wheel.
The current waveform shown in FIG. 3, which is known from the related art, was also generated for the color wheel 10 shown in FIG. 1. Only two commutations were set per image repetition here, however, from which a mean lamp frequency of 60 Hz results. The position of the commutations was again selected in this case so that the resulting current waveform is symmetrical. In the case of the current waveform shown in FIG. 3, it is to be considered that SP1 becomes an MP in the next half wave, which is then subjected to a commutation after this current increase. In principle, there are only two different current increases: a short one of 0.63 ms length and a long one of 1.28 ms length. Depending on whether or not a commutation follows, it is an MP or just a simple current increase SP.
The current waveforms shown in FIG. 2 and FIG. 3 fulfill the specifications of a DLP projector producer with respect to the useful light. Nonetheless, undesired electrode burning back is displayed upon the use of both current waveforms, so that only a comparatively short service life can be expected.
FIG. 4 shows a further current waveform known from the related art. It has a region 1, in which the illustrated half wave is 8.33 ms long. The associated full wave would accordingly be 16.67 ms long. The first frequency is accordingly 60 Hz in the present case. In the region 2, the first half wave is 4.40 ms long, and the associated full wave would be 8.80 ms long. The frequency is accordingly 1/8.80 ms=113.60 Hz. The second half wave in the region 2 is 3.94 ms long. The associated full wave would accordingly be 7.88 ms. The frequency would accordingly be 127 Hz. The modulation factor, calculated from the ratio of the mean frequency of the second region, the so-called second frequency, to the first frequency, is accordingly 2.0.
Such a current waveform accordingly has a moderate modulation factor and results from necessity, because no other matching commutation pattern was feasible for the color wheel predefined in FIG. 1.
A further current waveform having a modulation factor not equal to 1 is known from DE 10 2010 039 221 A1, which was published later, see FIG. 2b therein. A modulation factor calculated according to the algorithm mentioned at the outset is 2.2 for the current waveform shown therein.
An operation of a DLP projector using a current waveform according to FIG. 4 does not show any significant improvement with respect to tendency to burn back and stabilization of the electrode tip position in comparison to the current waveforms shown in FIG. 2 and FIG. 3, unfortunately. This will be discussed in greater detail hereafter with reference to FIGS. 6 and 7.